Tunable mass damper assembly for a rotor blade

ABSTRACT

A tunable mass damper assembly is attachable to a rotor blade. The tunable mass damper assembly comprises a base configured to be attached to the rotor blade and a pendulum mass structure movably attached to the base and configured to move relative to the base in accordance with a rotational speed of the rotor blade about a rotor axis. The pendulum mass structure is configured to reduce vibratory forces of the rotor blade induced by a rotation of the rotor blade about the rotor axis. An entirety of the pendulum mass structure being configured to be contained within and enclosed by the rotor blade.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Contract No.W911W6-19-9-0005, awarded by the U.S. Army. The government has certainrights in the invention.

FIELD

The present application relates generally to tunable mass damperassemblies, such as pendulum assemblies, for a rotor blade of a rotorsystem of a rotary wing aircraft.

BACKGROUND

Asymmetric lift of helicopter blades in forward flight createssignificant vibrations, which is a frequent problem during the operationof aircraft, in particular helicopters. Vibration is detrimental to,decreases the life of, and increases the fatigue of variousfatigue-loaded components of the aircraft, such as the rotor system andthe airframe. Furthermore, vibration is also detrimental to the overallperformance of the aircraft, and adversely affects the human flightcrew. Accordingly, it would be beneficial to attenuate or eliminatethese vibrations to extend the operational life of the aircraft andimprove ride characteristics.

SUMMARY

Typical dampers provide a simple weighted pendulum mechanism that isattached to the external surface of helicopter main rotor blades. Suchdampers are not aerodynamic and thus increase the aerodynamic drag ofthe rotor system, and decrease the aerodynamic performance of the rotorblade. However, reducing aerodynamic drag and increasing aerodynamicperformance are particularly important as the forward flight speed ofthe aircraft increases (e.g., for advanced rotorcraft). The exemplaryembodiments set forth herein address these and other issues.

Various embodiments provide for a tunable mass damper assembly that isattachable to a rotor blade. The tunable mass damper assembly comprisesa base configured to be attached to the rotor blade and a pendulum massstructure movably attached to the base and configured to move relativeto the base in accordance with a rotational speed of the rotor bladeabout a rotor axis. The pendulum mass structure is configured to reducevibratory forces of the rotor blade induced by a rotation of the rotorblade about the rotor axis. An entirety of the pendulum mass structurebeing configured to be contained within and enclosed by the rotor blade.

Various other embodiments provide for a tunable mass damper assemblythat is attachable to a rotor blade. The tunable mass damper assemblycomprises a base configured to be attached to the rotor blade and apendulum mass structure movably attached to the base and configured tomove relative to the base depending on a rotational speed of the rotorblade about a rotor axis. The pendulum mass structure is configured toreduce vibratory forces of the rotor blade induced by a rotation of therotor blade about the rotor axis. The tunable mass damper assembly has acontoured shape in a direction between a leading edge and a trailingedge of the rotor blade.

These and other features (including, but not limited to, retainingfeatures and/or viewing features), together with the organization andmanner of operation thereof, will become apparent from the followingdetailed description when taken in conjunction with the accompanyingdrawings, wherein like elements have like numerals throughout theseveral drawings described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a side view of an aircraft according to one embodiment.

FIG. 1B is a perspective view of the aircraft of FIG. 1A.

FIG. 2 is a perspective view of a portion of a rotor system that may beused within the aircraft of FIG. 1A, according to one embodiment.

FIG. 3A is a perspective view of a portion of the rotor system of FIG.2, according to one embodiment.

FIG. 3B is a cross-sectional view of a portion of FIG. 3A.

FIGS. 3C-3D are cross-sectional views of a portion of FIG. 3A with apendulum mass structure in different positions.

FIG. 4 is a cross-sectional view of a rotor system with a damperassembly according to another embodiment.

FIGS. 5A-5B are cross-sectional views of a portion of a rotor systemwith a pendulum mass structure in different positions according toanother embodiment.

FIG. 6 is a cross-sectional view of a portion of a rotor systemaccording to another embodiment.

FIG. 7A is a partial cross-sectional view of a portion of a rotor systemaccording to another embodiment.

FIG. 7B is a cross-sectional view of FIG. 7A.

FIGS. 7C-7E is the rotor system of FIG. 7A with a pendulum massstructure in different positions.

FIG. 8A is a perspective view of a rotor system according to anotherembodiment.

FIG. 8B is a perspective view of a rotor blade and a damper assembly ofthe rotor system of FIG. 8A.

FIGS. 8C-8D are perspective views of portions of FIG. 8B.

FIG. 8E is a perspective view of the damper assembly of FIG. 8B.

FIG. 8F is a top view of a portion of the FIG. 8B.

FIG. 8G is a cross-sectional view of a portion of FIG. 8B.

FIGS. 9A-9C are side view of the rotor system of FIG. 8B with a pendulummass structure in different positions.

FIG. 10A is a perspective view of a rotor system according to anotherembodiment.

FIG. 10B is a perspective view of a rotor blade and a damper assembly ofthe rotor system of FIG. 10A.

FIG. 10C is a perspective view of a portion of FIG. 10B.

FIG. 10D is a cross-sectional view of a portion of FIG. 10B.

FIG. 10E is a top view of a portion of the FIG. 10B.

FIGS. 11A-11C are side view of the rotor system of FIG. 10B with apendulum mass structure in different positions.

FIG. 12 is a perspective view of a portion of a rotor system accordingto another embodiment.

FIG. 13 is a perspective view of a portion of a rotor system accordingto another embodiment.

FIG. 14 is a perspective view of a damper assembly according to anotherembodiment.

DETAILED DESCRIPTION

Referring to the figures generally, various embodiments disclosed hereinrelate to various internal and external tunable (or tuned) mass damperassemblies that are attachable to a rotor blade of an aircraft toisolate, counteract, and dampen vibration of the aircraft at the sourceof the vibration (i.e., at the rotor blade), thereby reducing thevibrations conveyed to the aircraft. By dampening the vibration, thevarious detrimental effects of vibration are greatly reduced, if noteliminated. Furthermore, due to the configurations of the various damperassemblies disclosed herein, the damper assemblies reduce or eliminatethe aerodynamic drag of the rotor system.

Rotor System

FIGS. 1A-1B illustrate an exemplary vertical takeoff and landing (VTOL)high speed compound or coaxial counter-rotating rigid rotary wingaircraft 10 (which may be, for example only, a helicopter or a varietyof other devices which include at least one rotor blade). The aircraft10 includes an aircraft body or airframe 14, a dual, counter-rotating,coaxial main rotor system 20, a translational thrust system 18, atransmission 16, and at least one engine 15 (which may be a gas turbineengine). The overall structure and configuration of the aircraft 10 mayhave a variety of different configurations, including but not limited tothe structures disclosed in U.S. Pat. No. 10,822,076, the entirety ofwhich is incorporated by reference. The airframe 14 is a non-rotatingframe (relative to the main rotor system 20 and the translational thrustsystem 18) and supports the main rotor system 20 and the translationalthrust system 18.

The main rotor system 20 is driven by the transmission 16 and rotatesabout a hub or rotor axis 11. The main rotor system 20 may be a coaxialrotor system that includes an upper rotor assembly 21 and a lower rotorassembly 22 as dual counter-rotating main rotors in a coaxialconfiguration. The upper rotor assembly 21 is positioned above the lowerrotor assembly 22. The upper rotor assembly 21 and the lower rotorassembly 22 are rotated about the same, single axis (i.e., the rotoraxis 11) and may include concentric shafts or rotor masts. The upperrotor assembly 21 and the lower rotor assembly 22 may be rotated inopposite directions and are timed and controlled to cancel out the nettorque on the other rotor assembly in real-time, thereby providing anet-zero torque about the airframe 14, increasing the stability of theaircraft 10, and increasing the hovering capabilities of the aircraft10. However, according to various other embodiments, the main rotorsystem 20 may only include one rotor assembly.

The main rotor system 20 includes a plurality of main rotor blades 30(e.g., a rotor blade spar). In particular, each of the upper rotorassembly 21 and the lower rotor assembly 22 includes a set of rotorblades 30, as well as a rotor unit or hub 28 to which each of the rotorblades 30 is attached. The rotor hub 28 is configured to rotate aboutthe rotor axis 11 (thereby rotating the rotor blades 30 about the rotoraxis 11), and the upper rotor assembly 21 or the lower rotor assembly 22is mounted to the rotor hub 28. As shown in FIG. 2, the rotor hub 28includes a rotor mast 29 and a plurality of extensions or projections27. The rotor mast 29 extends upwardly along and around the rotor axis11 and is rotated about the rotor axis 11 relative to the airframe 14 torotate the rotor hub 28 (and thus the rotor blades 30) about the rotoraxis 11. The projections 27 (which may be configured as sets ofprojections) each correspond to one of the rotor blades 30.

The translational thrust system 18 provides translational thrustgenerally parallel to an aircraft longitudinal axis 12 (that extendsalong the length of the aircraft 10). The translational thrust system 18may be selected from one of a plurality of propeller systems including,but not limited to a pusher propeller, a tractor propeller, a nacellemounted propeller, etc. In the example of FIGS. 1A-1B, the translationalthrust system 18 includes an auxiliary propulsor 19. In an embodiment,the auxiliary propulsor 19 is a pusher propeller system with a propellerrotational axis oriented substantially horizontal and parallel to theaircraft longitudinal axis 12 to provide thrust for high speed flight.The translational thrust system 18 may be driven through a main gearbox17 which also drives the main rotor system 20.

The transmission 16 includes the main gearbox 17 driven by the one ormore engines 15. The main gearbox 17 and the engines 15 may be mountedon the airframe 14 of the aircraft 10. Thus, the main gearbox 17 andengines 15 form part of the overall assembly including airframe 14. Inthe case of a rotary wing aircraft, the main gearbox 17 may beinterposed between the one or more engines 15, the main rotor system 20,and the translational thrust system 18. In one embodiment, the maingearbox 17 is a split torque gearbox which carries torque from theengines 15 through a multitude of drivetrain paths. Although aparticular rotary wing aircraft configuration is illustrated anddescribed in the disclosed non-limiting embodiment, other configurationsand/or machines with rotor systems are within the scope of the presentdisclosure. It is to be appreciated that while the description hereinrelates to a rotary wing aircraft with a dual coaxial counter-rotatingrotor system, the disclosure herein may be as readily applied to otherrotor systems, such as turboprops, tilt-rotors, and tilt-wing aircraft,or a conventional single rotor system.

Rotor Blade

Each of the rotor assemblies 21, 22 may include any number of rotorblades 30, such as three or four rotor blades 30, that rotate with therotor hub 28 about the rotor axis 11. Each of the rotor blades 30 ismounted to the respective rotor hub 28 of the rotor assembly 21, 22 andare circumferentially spaced apart from each other. As described furtherherein, an internal or external tunable mass damper assembly 50 isattached to each of the rotor blades 30.

As shown in FIG. 2, each of the rotor blades 30 includes a rotor bladebody 36 and a blade neck 38. The blade neck 38 is configured to directlyattach to and extend radially outwardly from the rotor hub 28. Theprojections 27 of the rotor hub 28 extend at least partially into aninner area of the blade neck 38. The blade body 36 directly attaches toand extends radially outwardly from the blade neck 38 and terminates atthe outboard end or tip of the rotor blade 30. The blade neck 38 mayoptionally extend into an inner area of the blade body 36. Optionally,the blade neck 38 and the blade body 36 may be two separate componentsthat are attachable (and removable and reattachable) to each other.Alternatively, the blade neck 38 and the blade body 36 may beconstructed as a single unitary piece or component that cannot beseparated without destruction.

The rotor blade 30 includes a leading edge 31 and a trailing edge 32that extend along both the blade body 36 and the blade neck 38. Theleading edge 31 is upstream edge of the rotor blade 30, and the trailingedge 32 is downstream of the rotor blade 30 in the rotational directionof travel of the rotor blade 30 about the rotor axis 11. The leadingedge 31 and the trailing edge 32 extend along the radial length of therotor blade 30 and are opposite each other.

The rotor blade 30 further includes a top side or portion 33 and abottom side or portion 34. The top portion 33 faces upward, away fromthe airframe 14. The bottom portion 34 faces downward, toward theairframe 14. The top portion 33 and the bottom portion 34 extend alongthe radial length of the rotor blade 30, are opposite each other, andextend between the leading edge 31 and the trailing edge 32. Similarly,the leading edge 31 and the trailing edge 32 each extend between the topportion 33 and the bottom portion 34.

The longitudinal or feathering axis 24 of the rotor blade 30 refers tothe axis about which the pitch angle of the rotor blade 30 is varied. Inparticular, the rotor blade 30 feathers or twists about its featheringaxis 24 about at least one bearing to change the pitch angle, whichchanges the lift and drag. For example, by increasing the pitch angle,the rotor blade 30 provides more lift. Conversely, by decreasing thepitch angle, the rotor blade 30 provides less lift. As shown in FIG. 2,the feathering axis 24 extends substantially perpendicular to the rotoraxis 11.

Tunable Mass Damper Assembly

According to various embodiments as shown in FIGS. 3A-14, the aircraft10 includes at least one pendulum absorber, pendulum mechanism, swingingtunable mass assembly, or tunable mass damper device or assembly 50(referred to herein as the “damper assembly 50”). By moving or pivotinga pendulum mass structure 60 of the damper assembly 50 vertically up anddown (as described further herein), the tunable mass damper assembly 50is configured and optimized or tuned to isolate, absorb, dissipate,mitigate, or dampen energy and vibrations of the aircraft 10 that may becaused the rotor blades 30, thereby improving the performance of theaircraft 10, reducing or eliminating rotor system 20 and airframe 14vibration, and increasing component life, crew comfort, and aircraftendurance.

Each of the tunable mass damper assemblies 50 is configured to attach toone the rotor blades 30 and rotates with the rotor blade 30 about therotor axis 11. Although only one rotor blade 30 is shown with the damperassembly 50 (see, for example, FIGS. 8A and 10A), a damper assembly 50may be positioned on or within each of the rotor blades 30. Furthermore,although the damper assembly 50 is shown with the main rotor blade 30,the damper assembly 50 may be used with other types of rotor blades.

As described further herein, the damper assembly 50 may be positionedand secured internally within or externally on the rotor blade 30, bothof which reduce or eliminate aerodynamic drag otherwise caused by adamper. For example, in embodiments in which the damper assembly 50 ispositioned internally within the rotor blade 30, the damper assembly 50does not cause any aerodynamic drag since the damper assembly 50 iscompletely enclosed by the rotor blade 30. In embodiments in which thedamper assembly 50 is positioned externally on the rotor blade 30, theexternal damper assemblies 50 have an aerodynamic configuration (asdescribed further herein) which reduces aerodynamic drag compared totypical dampers. As described further herein, the various embodimentsdisclosed herein provide a variety of different configurations, each ofwhich achieve the desired mass/arm-length relationship within or on theconfined available space of the rotor blade 30.

By attaching the damper assembly 50 to a specific location on the rotorblade 30, the vibratory forces conveyed to the rotor hub 28 and airframe14 of the aircraft 10 (from the rotor blade 30) can be reduced. Incontrast to typical dampers, the damper assembly 50 is configured to bepositioned on (or within) and used with (or within) a rotor blade 30that is part of a coaxial and/or rigid rotor system (such as the rotorsystem 20). The rotor system 20 may optionally be fully rigid.

In rigid rotor systems, the rotor blades 30 move vertically relative tothe rotor hub 28 significantly less (if at all) compared to articulatedrotor systems in which the rotor blades are configured to move or “flap”vertically upward and downward relative to the rotor hub as a result ofaerodynamic loads. For example, in rigid rotor systems, the rotor blades30 may be statically attached to the rotor hub 28 and cannot articulaterelative to the rotor hub 28 (which may increase the loads andvibrations), whereas in articulated rotor system, the rotor blades maybe hingedly (and movably) attached to the rotor hub. Articulated rotorsystems are used primarily in lower speed flight regimes, whereaerodynamic drag is not as significant a factor and an externalconfiguration may be acceptable. Comparatively, in rigid rotor systems,which are used for high speed flight regimes, aerodynamic drag is moreof a concern. As described further herein, the various embodiments ofthe damper assembly 50 reduce the aerodynamic drag compared to typicaldamper assemblies and therefore can be used with rigid rotor systems(e.g. fully rigid systems).

As shown in the various embodiments (see, for example, FIGS. 3B and 8C,among others), each of the damper assemblies 50 include a base 52 and apendulum mass structure 60. The mounting bracket or base 52 isconfigured to be rigidly or movably attached or secured to the rotorblade 30. In particular, according to various embodiments as shown, forexample, in FIGS. 7B and 8C, the base 52 is rigidly secured to the rotorblade 30. According to various other embodiments as shown, for example,in FIG. 3B, the base 52 is movably secured to the rotor blade 30, asdescribed further herein. As also described further herein, depending onwhether the damper assembly 50 is internal or external to the rotorblade 30, the base 52 is configured to attach to the rotor blade 30 andbe positioned along either along an inner surface or an outer surface ofthe rotor blade 30.

The pendulum mass assembly or structure 60 is movably attached to thebase 52 and is configured to move relative to the base 52 depending on arotational speed of the rotor blade 30 about the rotor axis 11. Thependulum mass structure 60 is configured to reduce vibratory forces ofthe rotor blade 30 by moving relative to the base 52. The pendulum massstructure 60 comprises a pendulum mass 70 and at least one pendulum arm80, both of which are movable relative to the base 52 (and may either bemovably or statically attached to each other). The pendulum massstructure 60 is oriented on or within the rotor blade 30 such that thependulum mass 70 is closer to the outermost radial end of the rotorblade 30 than the pendulum arm 80, and the pendulum arm 80 is closer tothe innermost radial end of the rotor blade 30 (and therefore closer tothe rotor hub 28) than the pendulum mass 70.

The cantilever or pendulum weight or mass 70 is a weight portion of thependulum mass structure 60 that is substantially heavier and greater inmass than the pendulum arm 80. The mass or weight of the pendulum mass70 can vary (according to various designs), depending on the specificnatural frequency of the aircraft 10 for the damper assembly 50 tocounteract. The pendulum mass 70 includes a first mass end 71 and asecond mass end 72 that are opposite each other in a direction along thepivot axis 64 (as described further herein). The first mass end 71 iscloser to the leading edge 31 of the rotor blade 30 (than the secondmass end 72), and the second mass end 72 is closer to the trailing edge32 of the rotor blade 30 (than the first mass end 71). Accordingly, thefirst mass end 71 is upstream, and the second mass end 72 is downstream(relative to each other) in the rotational direction of travel of therespective rotor blade 30 about the rotor axis 11.

A first mass side of the pendulum mass 70 is directly attached to thesecond arm end 82 of the pendulum arm 80 (as described further herein),and a second mass side (that is opposite the first mass side) is notcontacting another structure when the pendulum mass structure 60 is notabutting one of the stops 58, 59 (as described further herein).Accordingly, when the pendulum mass structure 60 is not abutting one ofthe stops 58, 59, the pendulum mass 70 is attached to the rest of theaircraft 10 only via the pendulum arm 80. Optionally, the damperassembly 50 (in particular the pendulum mass structure 60) may includeonly a single pendulum mass 70.

The swing or pendulum bracket or arm 80 extends from the first mass sideof the pendulum mass 70 and spaces the pendulum mass 70 radially fromthe base 52 by a distance (which is correlated to the length of thependulum arm 80). The length of the pendulum arm 80 can vary (accordingto various designs), depending on the specific natural frequency of theaircraft 10 for the damper assembly 50 to counteract. As shown, forexample, in FIGS. 6, 7B, 8G, and 14, the pendulum arm 80 includes afirst arm end 81 and a second arm end 82. The first arm end 81 of thependulum arm 80 is movably, hingedly, pivotably, or rotatably attachedto the base 52 via a shaft, bolt, or hinge pin that extends along thepivot axis 64. The second arm end 82 of the pendulum arm 80 is directlyattached to the pendulum mass 70. Optionally, the motion of the pendulummass structure 60 (in particular, the motion of the pendulum arm 80) maybe resisted by a load to control the movement of the pendulum massstructure 60.

According to various embodiments, the pendulum mass 70 and the secondarm end 82 of the pendulum arm 80 are either movably, hingedly,pivotably, or rotatably attached together (via a shaft, bolt, or hingepin that extends substantially parallel to the pivot axis 64 and asshown in various embodiments, such as FIGS. 7A-7E) or statically orrigidly attached together (as shown in various embodiments, such as FIG.14). Furthermore, the pendulum mass 70 and the pendulum arm 80 may betwo separate components that are attachable to each other or may beconstructed as a single unitary piece or component that cannot beseparated without destruction.

The pendulum mass structure 60 is configured to be positioned along andmove vertically along a vertical plane that is substantially parallel tothe feathering axis 24 of the rotor blade 30 and the rotor axis 11 suchthat the pendulum mass structure 60 moves substantially perpendicular tothe feathering axis 24 of the rotor blade 30. In particular, thependulum arm 80 extends substantially parallel to the feathering axis 24and rotates about a pivot axis 64 that is substantially perpendicular tothe feathering axis 24 and the rotor axis 11, as shown in FIG. 4.Optionally, the pendulum mass structure 60 may be centered on the rotorblade 30 (e.g., centered between the leading edge 31 and the trailingedge 32) and aligned with the feathering axis 24 (as shown in FIG. 8B)or offset from the center and the feathering axis 24 and positionedalong either the leading edge 31 or the trailing edge 32 (as shown inFIG. 13). The damper assembly 50 is positioned such that the base 52 ispositioned radially inward from the pendulum mass structure 60 (inparticular from the pendulum mass 70), and the pendulum arm 80 extendradially between the base 52 and the pendulum mass 70.

To tune the damper assembly 50 to the specific natural frequency of theaircraft 10 (and/or the rotor blade 30), the operating frequencies ofthe aircraft 10 can be predicted and measured (in-flight) to the extentthat the physical structure and performance parameters of the rotorblade 30 are known or may be estimated. These operating frequencies aredirectly correlated to vibrations (e.g., the natural frequency)experienced by the rest of the aircraft 10. The mass of the pendulummass 70 and the length of the pendulum arm 80 (and their ratio) aretuned to this natural frequency which is specific to the rotor blade 30or aircraft 10.

To counteract the vibrations of the aircraft (that are created by therotor blades 30), the damper assembly is attached to the rotor blade 30,which is a vibrating structure. When the damper assembly 50 is excitedby the vibration of the rotor blade 30, the damper assembly 50 induces avibratory force that acts out-of-phase to the vibratory force of therotor blade 30, thereby reducing the overall vibration.

Range of Motion of the Damper Assembly

FIGS. 7C-7E, 9A-9C, and 11A-11C show the range of motion of the damperassembly 50. The damper assembly 50 and/or the rotor blade 30 mayinclude at least one stop (preferably a lower stop 58 and an upper stop59) that limits the downward and upward travel of the pendulum massstructure 60, respectively, through the range of motion as the rotorblade 30 (with the pendulum mass structure 60) is either at rest orrotates about the rotor axis 11. The pendulum mass structure 60 includesa first portion 61 and a second portion 62 that are configured to abutthe lower stop 58 and the upper stop 59 in the lowermost and uppermostpositions of the pendulum mass structure 60, respectively. Accordingly,the pendulum mass 70 may vertically swing or pitch freely (or withrestrained motion) as the first and second portions 61, 62 of thependulum mass structure 60 move between the lower stop 58 and the upperstop 59.

Depending on the embodiment and as described further herein, the firstportion 61 and the second portion 62 of the pendulum mass structure 60may refer to or include opposite sides of the pendulum arm(s) 80,opposite sides of the pendulum mass 70, opposite sides of an armextension 83 of the pendulum arm 80, and/or opposite sides of aconnector 57. Furthermore, depending on the embodiment, the lower stop58 and the upper stop 59 may refer to or include an upper surface and alower surface of the base 52, the respective inner surfaces of thebottom portion 34 and the top portion 33 of the rotor blade 30, innersurfaces of the base 52, and/or respective outer surface of the bottomportion 34 and the top portion 33 of the rotor blade 30.

For example, according to embodiment as shown in FIGS. 7C-7E and asdescribed further herein, the base 52 (and/or the inner surface of therotor blade 30) includes the lower stop 58 and the upper stop 59 thatare below and above the pendulum mass structure 60 and abut the firstportion 61 and the second portion 62 in the lowermost and uppermostpositions, respectively. According to another embodiment as shown inFIGS. 9A-9C and as described further herein, the base 52 includes thelower stop 58 and the upper stop 59 that abut the first portion 61 andthe second portion 62 in the lowermost and uppermost positions,respectively. According to another embodiment as shown in FIGS. 11A-11Cand as described further herein, the rotor blade 30 includes the lowerstop 58 and the upper stop 59 that abut the first portion 61 and thesecond portion 62 in the lowermost and uppermost positions,respectively.

FIGS. 7C-7E, 9A-9C, and 11A-11C show exemplary ranges of motion of thependulum mass structure 60 as at least a portion of the pendulum massstructure 60 pitches about at least one pivot axis 64 relative to thebase 52 to dampen out the vibration. When the rotor system 20 is notspinning and at rest (as shown in FIGS. 7C, 9A, and 11A), the firstportion 61 of the pendulum mass structure 60 rests on top of or abutsthe lower stop 58 due to gravity, and the lower stop 58 prevents thependulum mass structure 60 from moving any further lower. Accordingly,FIGS. 7C, 9A, 11A show the maximum down stroke and the lower range ofmotion and lowermost position of the pendulum mass structure 60.

When the rotor system 20 begins spinning in-flight (as shown in FIGS.7D-7E, 9B-9C, and 11B-11C), a centrifugal force field will cause thependulum mass 70 and the pendulum arm 80 to swing radially outwardly(toward the outboard end of the rotor blade 30) into an extendedposition and approach (or surpass) being positioned in a horizontalplane parallel to the feathering axis 24 of the rotor blade 30 to movesubstantially perpendicular to the feathering axis 24. FIGS. 7D, 9B, and11B show the fully extended position in which the first and secondportions 61 and 62 of the pendulum mass structure 60 are centeredbetween the lower stop 58 and the upper stop 59, and the pendulum arm 80is approximately horizontal (and parallel to the feathering axis 24).

FIGS. 7E, 9C, and 11C show the maximum up stroke and the upper range ofmotion and uppermost position of the pendulum mass structure 60. In thisposition, the second portion 62 of the pendulum mass structure 60 abutsthe upper stop 59, and the upper stop 59 prevents the pendulum massstructure 60 from moving further upward during movement, therebyrestraining movement beyond a given position.

Internal Tunable Mass Damper Assembly

According to various embodiments shown in FIGS. 3A-7E, the tunable massdamper assembly 50 is an internal tunable mass damper assembly. With aninternal damper assembly 50, an entirety of the pendulum mass structure60 (and optionally the entirety of the internal damper assembly 50,including an entirety of both the base 52 and the pendulum massstructure 60) is configured to be contained, positioned internallywithin, and completely enclosed by the rotor blade 30.

By positioning the damper assembly 50 internally within the rotor blade30, most or all of the aerodynamic drag that would otherwise be causedby the damper assembly 50 is avoided (i.e., there is no drag penalty asa result of the internal damper assembly 50). This is particularlyimportant for coaxial aircraft where the main rotor blade count isdoubled, which would multiply and significantly increase any aerodynamicdrag impact as a result from a damper. This is in contrast to damperswhich are not configured to be positioned internally within a rotorblade and instead are externally attached to the outer mold line (“OML”)of the rotor blade, which produces aerodynamic drag (unlike the internaldamper assembly 50). For a high speed aircraft in particular, thisincrease in aerodynamic drag is not an acceptable performance loss.Furthermore, by positioning the damper assembly 50 internally, all ofthe components of the damper assembly 50 are enclosed, therebypreventing any components from exiting the aircraft 10 in the event offailure.

The rotor blade designs of existing aircraft may not have sufficientinternal volume for a conventional damper to properly function withinthe rotor blade. For example, in articulated rotor systems (in which therotor blades are hingedly attached to the rotor hub), the inboardlocations of the rotor blade do not need a large spar because themoments are relatively low and the added size would only add to theweight of the rotor system. In typical rigid rotor systems, the size ofthe blade profile is minimized to use the smallest geometry acceptableto manage the higher bending moments, while optimizing the aerodynamicbehavior. However, it has been found that having a larger spar (blade)geometry provides a superior overall system with the rigid rotor. Thelarger blade geometry allows the damper assembly 50 to be positionedinternally within the rotor blade 30, in particular for flightapplications where an external damper would significantly worsen theperformance of the aircraft.

As described further herein, the internal damper assembly 50 may beattached to the rotor blade 30 in a variety of different ways. Forexample, as shown in FIGS. 3B-4, the rotor blade 30 is movably secured(via the center block 44, for example) to the rotor blade 30 such thatthe damper assembly 50 (including both the base 52 and the pendulum massstructure 60) does not pitch with the rotor blade 30 when the rotorblade 30 rotates about the feathering axis 24 to change a pitch angle.Alternatively, as shown in FIGS. 7A-7E, the base 52 of the damperassembly 50 is directly attached to the rotor blade 30 (with the base 52statically or rigidly secured to the rotor blade 30) such that thedamper assembly 50 (including both the base 52 and the pendulum massstructure 60) pitches with the rotor blade 30 when the rotor blade 30rotates about the feathering axis 24 to change a pitch angle.

According to one embodiment shown in FIGS. 3A-3D, the rotor system 20includes a composite or tension strap 41, an inboard bearing block 42, acenter block 44, an outboard bearing block 46, and an elastomeric trustbearing 48 along each of the sets of projections 27 of the rotor hub 28and within each rotor blade 30. The tension strap 41 reacts thecentrifugal force back into the rotor hub 28. The tension strap 41 isattached to each of the projections 27 (within a set of projections 27corresponding to one rotor blade 30) and extends around a radial end ofthese various components (which includes the damper assembly 50) thatare surrounded by the two projections 27. The overall structure andconfiguration of the tension strap 41 may have a variety of differentconfigurations, including but not limited to the configurationsdisclosed in U.S. Pat. No. 10,618,630, the entirety of which isincorporated by reference (including for the depicted structures anddescriptions thereof therein).

As shown in FIG. 3A, the inboard bearing block 42, the center block 44,the outboard bearing block 46, and the elastomeric trust bearing 48 areradially aligned with each other, attached to each other via bearings,at least partially surrounded by and positioned between the twoprojections 27, and positioned radially between the tension strap 41 andthe center portion of the rotor hub 28. The inboard bearing block 42,the center block 44, the outboard bearing block 46, and the elastomerictrust bearing 48 may be positioned within the blade neck 38 of the rotorblade 30. The inboard bearing block 42 is positioned radially inwardfrom the center block 44 and the outward bearing block 46. The centerblock 44 is positioned radially between the inboard bearing block 42 andthe outboard bearing block 46. The outboard bearing block 46 ispositioned radially outward from the center block 44 and the inboardbearing block 42. The elastomeric trust bearing 48 is positionedradially outward from the outboard bearing block 46 and may becompressible.

As shown in FIGS. 3C-3D, the inboard bearing block 42 and the outboardbearing block 46 each statically attach to and rotate with the rotorblade 30 as the rotor blade 30 pitches about the feathering axis 24. Thecenter block 44 is rotatably attached to the inboard bearing block 42and the outboard bearing block 46 along its radial ends (via bearings)such that the rotor blade 30, the inboard bearing block 42, and theoutboard bearing block 46 rotate along the feathering axis 24 relativeto the center block 44 (and anything positioned within the center block44).

As shown in FIGS. 3B-3D, the damper assembly 50 is positioned and housedwithin the center block 44. Accordingly, the rotor blade 30, the inboardbearing block 42, and the outboard bearing block 46 pitch about thefeathering axis 24 relative to both the center block 44 and the damperassembly 50 (and the center block 44 and the damper assembly 50 of FIGS.3B-3D do not pitch with the rotor blade 30 as the rotor blade 30 pitchesor rotates about the feathering axis 24). The base 52 of the damperassembly 50 is statically attached to an inner portion of the centerblock 44, and the pendulum mass structure 60 (including the pendulummass 70 and the pendulum arm 80) is rotatably attached to the base 52,as described further herein. FIGS. 3C-3D show how the pendulum massstructure 60 pitches about the pivot axis 64 relative to the base 52 todampen out the vibration.

FIG. 4 shows another embodiment of an internal damper assembly 50 thatis also positioned within the center block 44. However, the base 52 mayoptionally be movably attached to an inside portion of the center block44.

FIGS. 5A-5B show another embodiment of an internal damper assembly 50 inwhich the damper assembly 50 is a cantilever absorber that fits withinthe relatively narrow space constraints of the rotor blade 30. Inparticular, the damper assembly 50 of FIGS. 5A-5B is positioned radiallyoutward from the outboard bearing block 46 and the elastomeric trustbearing 48, in a relatively smaller region of the rotor blade 30 (suchas the rotor blade body 36, rather than the blade neck 38). The damperassembly 50 of FIGS. 5A-5B is relatively narrow to avoid contact withthe inner surfaces of the rotor blade 30. The pendulum mass 70 isstatically or rigidly attached to the second arm end 82 of the pendulumarm 80. Rather than being attached to the base 52 via a bolt, thependulum arm 80 is a cantilever to allow the pendulum mass 70 to havespring cantilever motion to dampen the vibration.

The damper assembly 50 is statically or rotatably attached to an outerradial side of the outboard bearing block 46 via the elastomeric trustbearing 48 and extends outwardly radially from the elastomeric trustbearing 48. The elastomeric trust bearing 48 may be rotatable ortwistable relative to the outboard bearing block 46. Accordingly, as therotor blade 30 (and thus the outboard bearing block 46) pitches aboutthe feathering axis 24, the damper assembly 50 may not pitch with therotor blade 30.

FIG. 6 shows another embodiment of an internal damper assembly 50 inwhich the damper assembly 50 is attached to the outboard bearing block46 via a pin or bolt 49. The bolt 49 attaches the base 52, the outboardbearing block 46 and the center block 44 together. The damper assembly50 of FIG. 6 is positioned radially outward from the outboard bearingblock 46 and outside of the center block 44 (but internally within therotor blade 30).

FIGS. 7A-7E show another embodiment of an internal damper assembly 50 inwhich the damper assembly 50 is a bar linkage pendulum. Theconfiguration of the damper assembly 50 of FIGS. 7A-7E has compactpackaging and is lightweight. The damper assembly 50 is positionedwithin and directly attached to the rotor blade 30, rather than beingattached to or positioned within the center block 44 or the bearingblocks 42, 46. Accordingly, the damper assembly 50 of FIGS. 7A-7Epitches with the rotor blade 30 as the rotor blade 30 pitches about thefeathering axis 24.

Instead of including a single pendulum arm 80, the damper assembly 50 ofFIGS. 7A-7E includes a plurality or set of pendulum arms 80, such as twopendulum arms 80. The two pendulum arms 80 (i.e., pendulum arm linkages)extend substantially parallel to each other between the pendulum mass 70and the base 52. Each of the pendulum arms 80 are rotatably attached toboth the base 52 and the pendulum mass 70 via their first arm end 81 andtheir second arm end 82, respectively, thereby maximizing the availableinternal space within the rotor blade 30 and creating four pivot points.The uppermost pendulum arm 80 is attached to and positioned alongrespective top portions of the pendulum mass 70 and the base 52. Thelowermost pendulum arm 80 is attached to and positioned along respectivebottom portions of the pendulum mass 70 and the base 52. Thisconfiguration allows the pendulum mass 70 to move approximately straightup and down (rather than pivoting) to dampen the vibration.

As shown in FIGS. 7C-7E, two pivot axes 64 (that are parallel andaligned with each other) extend through the joints between each of thependulum arms 80 and their respective connection points to the base 52.As shown in FIGS. 7C-7E, the two pendulum arms 80 move in parallelrelation to each other as they rotate about their respective pivot axes64 to move the pendulum mass 70 vertically up and down relative to thebase 52 to dampen out the vibration.

The base 52 (and thus the damper assembly 50) does not extend throughthe walls of the rotor blade 30, but instead is positioned completelywithin the rotor blade 30. However, the base 52 of the damper assembly50 is firmly and robustly attached to the rotor blade 30. In particular,as shown in FIGS. 3A-3B, the base 52 includes a pivot support structureor structural mount 51 and a plurality of (preferably two) mount pads53. The structural mount 51 provides a rigid structure for the pendulumarms 80 to pivotably attach to. The two mount pads 53 are positioned onopposite ends of the structural mount 51, between the structural mount51 and an inner surface of rotor blade 30 (in particular, inner surfacesalong the top portion 33 and the bottom portion 34 of the rotor blade30. The mount pads 53 securely attach (via, for example, a frictionalfit) the rest of the damper assembly 50 to the rotor blade 30.

As shown in FIGS. 7C-7E, the pendulum mass structure 60 is pivotablerelative to the base 52 to dampen the vibration, as described furtherherein. In the embodiment shown in FIGS. 7C-7E, the first portion 61 ofthe pendulum mass structure 60 (that abuts the lower stop 58 in thelowermost position as shown FIG. 7C) is a lower surface of the lowermostpendulum arm 80. The second portion 62 of the pendulum mass structure 60(that abuts the upper stop 59 in the uppermost position as shown FIG.7E) is an upper surface of the uppermost pendulum arm 80. Furthermore,the lower stop 58 corresponds to an upper surface of a lower portion ofthe base 52, and the upper stop 59 corresponds to a lower surface of anupper portion of the base 52.

Alternatively or additionally, the pendulum mass 70 may be configured toabut the inner surfaces of the rotor blade 30 (or the base 52) in itslowermost and uppermost positions. Accordingly, the lower surface of thependulum mass structure 60 may correspond to the first portion 61 of thependulum mass structure 60, and the upper surface of the pendulum massstructure 60 may correspond to the second portion 62 of the pendulummass structure 60. The upper inner surface of the bottom portion 34 ofthe rotor blade 30 may correspond to the lower stop 58, and the lowerinner surface of the top portion 33 of the rotor blade 30 may correspondto the upper stop 59.

External Tunable Mass Damper Assembly

According to various embodiments shown in FIGS. 8A-14, the tunable massdamper assembly 50 is an external tunable mass damper assembly such thatat least a portion of (or the entirety of) the damper assembly 50 ispositioned outside of the rotor blade 30. To reduce or mitigate anyaerodynamic drag as a result of the external damper assembly 50, theexternal damper assembly 50 has a contoured, aerodynamic shape in adirection between the leading edge 31 and the trailing edge 32 of therotor blade 30. In particular, the pendulum mass 70 has an aerodynamicshape extending between the first mass end 71 (which is the leading endof the pendulum mass 70) and the second mass end 72 (which is thetrailing end of the pendulum mass 70).

Conventional aircraft with pendulum devices are articulated orsemi-rigid. The external damper assemblies 50 are superior toconventional dampers at least in that the external damper assemblies 50may be tailored for and used with a rigid (e.g., hingeless) rotor systemand may be mirrored on each of the coaxial rotors. Furthermore, sincethe external damper assemblies 50 have an aerodynamic shape, the presentexternal damper assemblies 50 result in less aerodynamic drag thanconventional dampers which are not aerodynamically shaped. The variousexternal damper assemblies 50 may be retrofit onto a rotor blade 30.

The damper assembly 50 may be secured to variety of different externalareas of the rotor blade 30. As shown in the various embodiments, thedamper assembly 50 may be installed on, positioned along, secured to,and attached to the inner radial end of the rotor blade body 36 (near orat the connection point between the rotor blade body 36 and the bladeneck 38). According to one embodiment as shown in FIGS. 8B-8C and 8F,the damper assembly 50 (in particular the pendulum mass structure 60)may be attached to, positioned along, and extend along the top portion33 of the rotor blade 30, with the pendulum mass structure 60(specifically the pendulum arm 80) substantially aligned with thefeathering axis 24. According to various other embodiments as shown inFIGS. 10A-13, the damper assembly 50 (specifically the pendulum massstructure 60) is attached to, positioned along, and extends along one ofthe leading edge 31 or the trailing edge 32 of the rotor blade 30, withthe pendulum mass structure 60 (and specifically the pendulum arm 80)substantially aligned with and parallel to the respective one of theleading edge 31 or the trailing edge 32.

FIGS. 8A-9C show one embodiment of an external damper assembly 50 thatis an “aero-bob” pendulum assembly. The first mass end 71 and the secondmass end 72 of the pendulum mass 70 (e.g., the “aero-bob mass”) areshaped differently from each other to form the contoured, aerodynamicshape of the damper assembly 50. In particular, the first mass end 71has a pointed end that transitions and curves radially therefrom to abulbous middle portion, which is the widest portion of the pendulum mass70 along its length. From the bulbous middle portion, the pendulum mass70 tapers gradually along its length toward the second mass end 72.Accordingly, the pendulum mass 70 has a “tear-drop” shape extending froma narrower end at the second mass end 72 to the bulbous middle portionand to the relatively more rounded first mass end 71. The pendulum mass70 is narrower along the second mass end 72 than the first mass end 71.

The pendulum arm 80 is attached to and extends radially inward from thebulbous middle portion of the pendulum mass 70. In particular, as shownin FIG. 8G, the pendulum arm 80 is fastened to the pendulum mass 70 viaattachment hardware (e.g., bolts). The pendulum arm 80 includes twoextensions that extend along upstream and downstream ends of the base52. A central pivot bolt or shaft extends through the two extensions andthe base 52 to rotatably connect the pendulum arm 80 to the base 52. Atleast one bushing may be positioned between the pivot bolt and thestructural mount 51 of the base 52.

As shown in FIG. 8D, the base 52 includes the structural mount 51 and amounting bracket 54. The mounting bracket 54 extends along the outersurface of the rotor blade 30 (for example, along the top portion 33)and is fastened or adhered to the rotor blade 30. The structural mount51 provides a rigid structure for the pendulum arms 80 to pivotablyattach to and extends from the mounting bracket 54 (in a direction awayfrom the rotor blade 30, which is vertically upwards in thisembodiment). The structural mount 51 includes two base extensions 51 a,51 b (as shown in FIGS. 8D-8E) that are radially aligned with eachother. The first base extension 51 a is positioned radially inwardrelative to the second base extension 51 b (and the second baseextension 51 b is positioned radially outward relative to the first baseextension 51 a). The base extensions 51 a, 51 b extend at an angle fromthe mounting bracket 54 and toward each other to form a hollow innertriangular area 55 with the mounting bracket 54. The pendulum arm 80 isrotatably attached to the structural mount 51 of the base 52 at theintersection of the two base extensions 51 a, 51 b.

The inner triangular area 55 (labeled in FIG. 9A) between the baseextensions 51 a, 51 b of the structural mount 51 and the mountingbracket 54 is hollow or open in a direction of travel of the rotor blade30 about the rotor axis 11 and in the direction extending between theleading edge 31 and the trailing edge 32 of the rotor blade 30. Thehollow inner triangular area 55 allows the base 52 to be moreaerodynamic and lightweight. Additionally, the hollow inner triangulararea 55 provides an area for an arm extension 83 of the pendulum arm 80to move within (as shown in FIGS. 9A-9C). In particular, as shown inFIGS. 8D and 9A-9C, the pendulum arm 80 includes an arm extension 83that is positioned at least partially within the inner triangular area55 (the arm extension 83 is not shown in all of the figures, such asFIG. 8E).

FIGS. 9A-9C show how the pendulum mass structure 60 rotates about thepivot axis 64 to move the pendulum mass 70 relative to the base 52 todampen out the vibration. The arm extension 83 is configured to abut theinner sides of each of the base extensions 51 a, 51 b as the pendulummass structure 60 moves about its entire range of motion (as shown inFIGS. 9A and 9C). Accordingly, the respective inner surfaces of the baseextensions 51 a, 51 b (that define a portion of the hollow innertriangular area 55) define the lower stop 58 and the upper stop 59 (asdescribed further herein). In particular, as shown in FIGS. 9A-9C, theinner surface of the first base extension 51 a defines the lower stop58, and the inner surface of the second base extension 51 b defines theupper stop 59. Furthermore, the opposite sides of the arm extension 83define the first portion 61 and the second portion 62 of the pendulummass structure 60 (as described further herein).

FIGS. 10A-11C show another embodiment of an external damper assembly 50,which may be a halo pendulum assembly. The damper assembly 50 comprisestwo pendulum mass structures 60 and two corresponding bases 52, where afirst (or leading edge) pendulum mass structure 60 a and a first base 52a are positioned along the leading edge 31 of the rotor blade 30 and asecond (or trailing edge) pendulum mass structure 60 b and a second base52 b are positioned along the trailing edge 32 of the rotor blade 30, asshown in FIG. 10C. The two pendulum mass structures 60 are aligned alongthe same pivot axis 64.

The damper assembly 50 further includes at least one connector 57 (e.g.,a halo bar) that statically connects the first and second pendulum massstructures 60 a, 60 b together such that the first and second pendulummass structures 60 a, 60 b (with the connector(s) 57) pivot together asone unit. Specifically, the connector 57 statically attaches to andextends between the two pendulum arms 80. Accordingly, the first andsecond pendulum mass structures 60 a, 60 b move congruently together asthey pivot about the pivot axis 64 (as shown in FIGS. 10D-10E). As shownin FIG. 12 and described further herein, however, the connectors 57 areremovable and uninstallable (as well as reattachable and reinstallable).

Optionally, as shown in FIG. 10D, the damper assembly 50 may include twoconnectors 57 that extend along opposite sides of the rotor blade 30 andattach to opposite sides of the two pendulum arms 80. For example, thedamper assembly 50 may include an upper or top connector 57 and a loweror bottom connector 57 that extend along the top portion 33 and thebottom portion 34 of the rotor blade 30, respectively. When the pendulumarms 80 are substantially parallel to the feathering axis 24, the topand bottom connectors 57 are vertically spaced apart from the topportion 33 and the bottom portion 34 of the rotor blade 30,respectively, along their entire length to allow each of the pendulummass structures 60 a, 60 b to move up and down relative to the rotorblade 30 about the pivot axis 64. By having the two connectors 57, thedamper assembly 50 has a balanced center of gravity about the pivot axis64.

To have an aerodynamic shape, each of the pendulum masses 70 of thefirst and second pendulum mass structures 60 a, 60 b has a contouredshape in the direction between the leading edge 31 and the trailing edge32 of the rotor blade 30. In particular, the pendulum masses 70 have acylindrical shape, where the rounded side of the pendulum mass 70 isconfigured to face upstream (and downstream) in the direction ofrotational travel about the rotor axis 11. The flat ends of the pendulummass 70 are approximately parallel to the direction of rotational travelabout the rotor axis 11. Furthermore, the connectors 57 have a curved orarched shape between their leading end and their trailing end to followthe contour of the rotor blade 30 and be aerodynamic.

As shown in FIG. 10D, each of the first and second bases 52 a, 52 bincludes the structural mount 51 and the mounting bracket 54, both ofwhich are described further herein. The structural mount 51, however,extends in a horizontal direction outward from the leading edge 31 orthe trailing edge 32 of the rotor blade 30 (rather than verticallyupward). The pendulum arm 80 is rotatably attached to the structuralmount 51 via a fastener (such as a central pivot bolt or shaft). Atleast one bushing may be positioned between the pivot bolt and thestructural mount 51 of the base 52.

FIGS. 11A-11C show how one of the pendulum mass structures 60 and theconnectors 57 pitch about the pivot axis 64 to move the pendulum mass 70relative to the base 52 to dampen out the vibration. The other pendulummass structure 60 moves congruently with and in the same manner as theshown pendulum mass structure 60. Opposite sides of each of theconnectors 57 are configured to abut the respective portions of outersurfaces of the top portion 33 and the bottom portion 34 of the rotorblade 30 as the pendulum mass structure 60 moves about its entire rangeof motion (as shown in FIGS. 11A and 11C). Accordingly, an area of theouter surface of the top portion 33 of the rotor blade 30 that isradially outward from the pivot axis 64 (relative to the feathering axis24) and an area of the outer surface of the bottom portion 34 of therotor blade 30 that is radially inward from the pivot axis 64 bothdefine lower stops 58 (as described further herein). Furthermore, anarea of the outer surface of the top portion 33 of the rotor blade 30that is radially inward from the pivot axis 64 and an area of the outersurface of the bottom portion 34 of the rotor blade 30 that is radiallyoutward from the pivot axis 64 both define upper stops 59 (as describedfurther herein).

For reference, the radial inward sides of the connectors 57 faceradially inward, toward the rotor hub 28 and the rotor axis 11. Theradial outward sides of the connectors 57 face radially outward, awayfrom the rotor hub 28 and the rotor axis 11 and toward the outboard tipof the rotor blade 30. Accordingly, a radial outward side of the topconnector 57 and a radial inward side of the bottom connector 57 definethe first portion 61 of the pendulum mass structure 60, and a radialinward side of the top connector 57 and a radial outward side of thebottom connector 57 define the second portion 62 of the pendulum massstructure 60 (as described further herein).

FIG. 12 shows another embodiment of an external damper assembly 50,which is similar to the embodiment shown in FIGS. 10A-11C, except thatthe connectors 57 are not included (or have been removed oruninstalled). Accordingly, the first and second pendulum mass structure60 a, 60 b are decoupled from each other and can move, pivot, or rotateabout the pivot axis 64 independently of each other. The first andsecond pendulum mass structures 60 a, 60 b (and their respective firstand second bases 52 a, 52 b) may optionally be part of two separatedamper assemblies 50 that are attached to the same rotor blade 30 (andaligned along the same pivot axis 64).

FIG. 13 shows another embodiment of an external damper assembly 50 thatis similar to the embodiment shown in FIG. 12, except for the shape ofthe pendulum masses 70. In particular, instead of having a flat,radially outward end, the pendulum masses 70 have a curved or domed end(that is opposite to the side of the pendulum mass 70 that connects tothe pendulum arm 80).

FIG. 14 shows another embodiment of a damper assembly 50 that may bepositioned in a variety of different areas along the rotor blade 30.Optionally, the damper assembly 50 may include a second mass 78 that isalso rotatable about the pivot axis 64.

Although each of the various aspects, features, components, andconfigurations are not separately described for each embodiment, each ofthe various embodiments (including both the internal and external damperassemblies 50) disclosed herein may have any of the aspects, features,components, and configurations of the other embodiments, except wherenoted otherwise.

As utilized herein, the terms “approximately,” “substantially,” andsimilar terms are intended to have a broad meaning in harmony with thecommon and accepted usage by those of ordinary skill in the art to whichthe subject matter of this disclosure pertains. The terms“approximately” and “substantially” as used herein refers to ±5% of thereferenced measurement, position, or dimension. It should be understoodby those of skill in the art who review this disclosure that these termsare intended to allow a description of certain features described andclaimed without restricting the scope of these features to the precisenumerical ranges provided. Accordingly, these terms should beinterpreted as indicating that insubstantial or inconsequentialmodifications or alterations of the subject matter described and claimedare considered to be within the scope of the invention as recited in theappended claims.

The terms “coupled,” “attached,” and the like as used herein mean thejoining of two members directly to one another. Such joining may bestationary (e.g., permanent) or moveable (e.g., removable orreleasable).

References herein to the positions of elements (e.g., “top,” “bottom,”“above,” “below,” etc.) are merely used to describe the orientation ofvarious elements in the FIGURES. It should be noted that the orientationof various elements may differ according to other exemplary embodiments,and that such variations are intended to be encompassed by the presentdisclosure.

It is important to note that the construction and arrangement of thevarious exemplary embodiments are illustrative only. Although only a fewembodiments have been described in detail in this disclosure, thoseskilled in the art who review this disclosure will readily appreciatethat many modifications are possible (e.g., variations in sizes,dimensions, structures, shapes and proportions of the various elements,values of parameters, mounting arrangements, use of materials,orientations, etc.) without materially departing from the novelteachings and advantages of the subject matter described herein. Forexample, the position of elements may be reversed or otherwise varied,and the nature or number of discrete elements or positions may bealtered or varied. The order or sequence of any process or method stepsmay be varied according to alternative embodiments. Other substitutions,modifications, changes and omissions may also be made in the design,operating conditions and arrangement of the various exemplaryembodiments without departing from the scope of the present invention.

What is claimed is:
 1. A tunable mass damper assembly attachable to arotor blade, the tunable mass damper assembly comprising: a baseconfigured to be attached to the rotor blade; and a pendulum massstructure movably attached to the base and configured to move relativeto the base in accordance with a rotational speed of the rotor bladeabout a rotor axis, the pendulum mass structure being configured toreduce vibratory forces of the rotor blade induced by a rotation of therotor blade about the rotor axis, and an entirety of the pendulum massstructure being configured to be contained within and enclosed by therotor blade.
 2. The tunable mass damper assembly of claim 1, wherein anentirety of the base is configured to be positioned within andcompletely enclosed by the rotor blade.
 3. The tunable mass damperassembly of claim 1, wherein the pendulum mass structure comprises apendulum mass and at least one pendulum arm, wherein a first arm end ofthe pendulum arm is attached to the base and a second arm end of thependulum arm is attached to the pendulum mass.
 4. The tunable massdamper assembly of claim 3, wherein the at least one pendulum arm isrotatably attached to both the pendulum mass and the base.
 5. Thetunable mass damper assembly of claim 1, wherein the pendulum massstructure is configured to be positioned along and move substantiallyperpendicular to a feathering axis of the rotor blade.
 6. The tunablemass damper assembly of claim 1, wherein the base is configured to berigidly secured to the rotor blade.
 7. The tunable mass damper assemblyof claim 1, wherein the base is configured to be movably secured to therotor blade such that the base and the pendulum mass structure do notrotate with the rotor blade when the rotor blade rotates about afeathering axis to change a pitch angle.
 8. The tunable mass damperassembly of claim 1, wherein the tunable mass damper assembly isconfigured to be positioned within a rotor blade that is part of acoaxial rotor system.
 9. The tunable mass damper assembly of claim 1,wherein the tunable mass damper assembly is configured to be positionedwithin a rotor blade that is part of a rigid rotor system.
 10. A tunablemass damper assembly attachable to a rotor blade, the tunable massdamper assembly comprising: a base configured to be attached to therotor blade; and a pendulum mass structure movably attached to the baseand configured to move relative to the base depending on a rotationalspeed of the rotor blade about a rotor axis, the pendulum mass structurebeing configured to reduce vibratory forces of the rotor blade inducedby a rotation of the rotor blade about the rotor axis, and the tunablemass damper assembly having a contoured shape in a direction between aleading edge and a trailing edge of the rotor blade.
 11. The tunablemass damper assembly of claim 10, wherein the pendulum mass structurecomprises a pendulum mass with a first mass end that is closer to theleading edge of the rotor blade and a second mass end that is closer tothe trailing edge of the rotor blade, wherein the first mass end and thesecond mass end of the pendulum mass are shaped differently to form thecontoured shape of the tunable mass damper assembly.
 12. The tunablemass damper assembly of claim 10, wherein the pendulum mass structure isconfigured to be positioned along and move substantially perpendicularto a feathering axis of the rotor blade.
 13. The tunable mass damperassembly of claim 10, wherein the pendulum mass structure is configuredto be positioned on top portion of the rotor blade.
 14. The tunable massdamper assembly of claim 10, wherein the pendulum mass structure isconfigured to be positioned along one of the leading edge and thetrailing edge of the rotor blade.
 15. The tunable mass damper assemblyof claim 10, wherein the rotor blade comprises a rotor blade body withthe leading edge and the trailing edge and a blade neck configured toattach to a rotor hub, wherein the tunable mass damper assembly isconfigured to be positioned along and secured to the rotor blade body.16. The tunable mass damper assembly of claim 10, further comprising asecond pendulum mass structure positioned along the trailing edge of therotor blade, wherein the pendulum mass structure is a first pendulummass structure that is positioned along the leading edge of the rotorblade.
 17. The tunable mass damper assembly of claim 16, furthercomprising a connector, wherein the first pendulum mass and the secondpendulum mass are statically connected together by the connector suchthat the first pendulum mass and the second pendulum mass move together.18. The tunable mass damper assembly of claim 17, wherein the connectorextends along a top portion or a bottom portion of the rotor blade. 19.The tunable mass damper assembly of claim 16, wherein the first pendulummass and the second pendulum mass are independently movable relative toeach other.
 20. The tunable mass damper assembly of claim 10, whereinthe tunable mass damper assembly is configured to be positioned on arotor blade that is part of at least one of a coaxial rotor system or arigid rotor system.